1. Field of the Invention
This invention relates to electrical contacts and thin film resistors which can be used with many types of circuitry, and which provide stability and resistance to interdiffusion between the materials to be connected.
2. Description of the Prior Art
Thin film resistors and electrical connections are well known in many prior art circuits. For instance, metallurgical connections have to be provided for electrical contacts to modules and chips in integrated circuitry.
In superconductive circuitry, resistors are often required, and it is also required to provide good contacts between superconductive transmission lines. For instance, circuitry using Josephson tunnelling devices often requires impedance terminations to transmission lines. One such impedance terminated circuit is shown in U.S. Pat. No. 3,758,795. In that patent, a Josephson tunnelling circuit is used to transfer current into a superconducting transmission line which is terminated by the proper resistive material. This transmission line can be used to provide control signals for other Josephson devices.
Provision of thin film resistors is very difficult and it is even more difficult in superconductive circuitry. Resistors must be stable with respect to storage, fabrication and thermal cycling. That is, the resistance value should not change more than a very small percent (perhaps 1%) in order to provide proper tolerance for circuitry. This stability must be present when the circuits are stored at any temperature, when the circuits are fabricated at elevated process temperatures (for instance, 100.degree. C), and during repeated thermal cycling between room temperatures and operating temperatures (i.e., cryogenic temperatures).
These resistors should also be reproducible; that is, the same resistance values should be obtainable over repeated fabrication runs. The resistance values at the operating temperatures of the circuits should be within a certain range each time circuitry is fabricated. This resistance range for reproducibility should be around 5% or less.
Reproducibility of resistance values is a difficult problem. Resistance of thin film materials depends on defects, phonon scattering, thickness, and surface effects. These defects include vacancies, dislocations, grain boundaries, impurities, etc. in the resistor material. To have reproducible resistors, it is necessary that the grain boundaries be reproducible each time the resistor is made. If the mean free path of electrons in the resistor is less than the grain size and less than the thickness of the material, then these effects will not be so important for the reproducibility of resistance values. However, electron scattering due to defects will be important and is a necessary factor to be considered in order to provide reproducible resistors.
The resistance range which can be obtained through the use of these thin film resistors is also an important consideration. The resistance range must be within desired values for proper design of superconductive circuits. For instance, for Josephson circuitry using lead alloy superconducting materials, resistance values of about 0.05-2 ohms/sq. may be desirable in many circuits. Here, the unit ohms/sq. is (resistance) (width)/length, which is equal to resistivity/thickness.
Further, in the design of Josephson tunnelling circuitry it is desirable to be able to use materials which can be reproducibly deposited at low substrate temperatures. For instance, ground planes comprising Nb are often utilized. These ground planes are covered with an oxide layer after which the tunnelling circuits (including resistors) are deposited. If the temperature at which the thin film resistors are deposited is too great (approximately 150.degree. C) penetration of oxygen from the oxide insulator into the Nb ground plane will occur. This will change the critical temperature of the superconducting ground plane and will also change the thickness of the oxide insulation layer. This change in thickness of the insulation layer will alter the characteristic impedance of transmission lines which are deposited over the insulating layer. This in turn will alter the electrical characteristics of the circuitry. Therefore, it is not desirable to use resistors comprising materials (such as refractory materials) which generally require high temperature deposition in order to be reproducible and stable.
A problem which occurs when contacts or resistors are made is that of interdiffusion. That is, the two materials which are contacted by interposed metallurgy can undergo interdiffusion by which the materials and the metallurgy diffuse into one another. This changes the material compositions and therefore changes the electrical resistance of the deposited resistor. When contacts are provided, interdiffusion can also alter the quality of the connection. Additionally, this diffusion problem may spread to affect the electrodes of tunnel devices which are connected to the resistor. This may cause a serious change in the tunnel junction properties of these devices, leading to further circuit problems. This is an especially difficult problem in large, densely packed arrays of Josephson junctions.
Interdiffusion can occur at room temperatures and at operating temperatures. In the case of superconductive circuitry, room temperature interdiffusion should be very small or approximately zero percent. Additionally, it is not expected that interdiffusion would be a serious problem for superconductive circuits at their operating temperatures, since these will be cryogenic temperatures. That is, because interdiffusion is a thermally activated process, operation of the circuits at cryogenic temperatures will not lead to serious interdiffusion problems.
Interdiffusion should also be minimal at the process temperatures at which the circuits are made as well as at the temperatures used for other steps, such as tailoring and stabilizing of electrical characteristics. For instance, resistors or contacts are often deposited early in the fabrication process by which superconductive circuits are made. This means that these materials must be able to withstand elevated temperatures, as for instance the higher temperatures which are used in subsequent annealing steps. Because of this, interdiffusion at the process temperatures must be controlled.
In addition to interdiffusion problems, resistors and contacts in circuitry often undergo stability changes due to the introduction of structural changes in the materials. For instance, defects in the materials may move into clusters or may migrate to grain boundaries. Also, recrystallization and grain boundary movement may occur in order to relieve stress in the materials, thereby altering the defect nature and density in the materials. These stresses can result due to thermal coefficient differences or due to the growth processes wherein defects, dislocations, vacancies, etc. may move in the material to relieve stresses.
Stability is very important in resistors to be used in low temperature circuitry (cryogenic circuitry). For instance, although solid solutions such as nichrome material, Pb-Ag, and Cu-Au are known as resistors, these solid solutions may not be useful at very low temperatures, such as 4.2.degree. K.
The prior art has not attempted to reduce the driving forces which cause interdiffusion between dissimilar materials which are joined in various circuitry. Additionally, the prior art has not directed its attention to the particular problems posed by superconductive circuitry, where the difference between the process temperatures, room temperatures, and operating temperatures are so large.
In addition to being useful for forming thin film resistors, the present invention can be used to provide interconnections between materials, and in particular provides good interconnections between circuit chips and modules having various electrical conductors thereon. In particular, embodiments will be shown for superconductive packaging between superconducting circuit chips and modules for providing electrical power to these chips. Additionally, embodiments will be shown for providing resistors in Josephson tunnelling device circuitry. These resistors can be comprised of compounds, or solid solutions, if the solid solutions are stable phases of the electrical lines which contact the resistor.
Accordingly, it is a primary object of the present invention to provide metallurgical interconnections which minimize interdiffusion between materials to be connected.
It is another object of this invention to provide techniques for forming resistive terminations to electrical conductors, where interdiffusion between the resistors and the electrical conductors is minimal.
It is another object to provide metallurgical connections to electrical conductors which are thermodynamically in equilibrium with these conductors, and between which interdiffusion is minimal.
It is a further object of this invention to provide resistors for superconductive circuitry, where interdiffusion is reduced.
It is a still further object of this invention to provide interconnection metallurgy for Josephson tunnelling circuits, which are in thermodynamic equilibrium with the circuits and in which interdiffusion problems are minimal.
It is a further object of this invention to provide improved interconnections between superconducting circuit chips and associated modules.
It is another object of this invention to provide interconnections between superconducting Josephson tunnelling circuit chips and circuit modules.
It is still another object of this invention to provide thin film resistors for electrical circuitry which are very stable.
It is a further object of this invention to provide thin film resistors by a layering technique.